The invention disclosed herein pertains to digital test equipment for verifying the proper operation of, and trouble shooting, digital subsystems that communicate over interconnect buses by means of standardized, multibit, serial communication words.
The invention primarily concerns apparatus and method for testing digital information exchange systems, such as found in digital avionics systems that are comprised of numerous line replaceable units (LRUs). Each LRU in the avionics system constitutes a replaceable electronics component, and each of these components communicates with others in the avionics system by means of transmitting and receiving digital information formatted in standardized multibit serial words that are recognized by the built-in intelligence of each LRU. For example, an LRU may sense the altitude of the aircraft and electronically formulate a digital serial word that, among other things, includes a binary representation of the engineering value of the altitude. Whereafter, the thusly formulated digital word is transmitted to another LRU which, for example, might be a digital readout on the instrument panel of the cockpit. There, the altitude data of the word is decoded and displayed on the readout in decimal notation or other pilot readable form.
While the LRU altimeter is but one example of a line replaceable subsystem, it will be appreciated that the actual avionics systems in large sophisticated aircraft of today involve many diverse LRUs which are interconnected by the digital buses to form a complex intelligence system. Because of the complexities and scale of such avionics systems, the aircraft industry has taken steps to standardize the digital words that are transmitted and received by the various LRUs so as to facilitate the interchageability of and maintenance of such components.
One aspect of this standardization is that the information transmitted and/or received by the LRUs is in the form of serial bit communication words, wherein each word includes a data field comprising the engineering or numeric content and/or discretes a label constituting another subgroup of bits and defining the function of the word (e.g., altitude) and certain other subgroups and single bits representing ancillary information. An example of such a standardized digital information word is specified in a publication prepared by Aeronautical Radio, Inc. of Annapolis, Md., entitled "Mark 33 Digital Information Transfer System (DITS) Specification No. 429-4," published Aug. 1, 1980, the contents of which are incorporated herein be reference. The communication word described in Specification No. 929-4, and known in the industry as the ARINC 429 word, is comprised of 32 bits in a standardized format which, as explained more fully hereinafter, can be used to represent over 300 different functions or parameters generated and/or received by commonly employed LRUs of an avionics system. Altitude was mentioned above, and other examples are latitude/longitude, ground speed, magnetic heading, wind speed, runway heading, vertical speed, frequency radio navigational signals, etc. The information carried in the various 32-bit communication words flow from one LRU port to one or more other LRU ports over a twisted, and preferably shielded, pair of wires in which only unidirectional serial flow of the digital word or words is permitted. Thus, when two LRUs require bidirectional communication, they will be coupled by at least two separate sets of twisted wire pairs.
Usually, a designated twisted wire pair serves as a common data but interconnecting a number of functionally related LRUs, hence, that bus will carry a number of different communication words, one for each of the parameters that is to be exchanged. For example, a single bus may carry communication words representing the parameters of latitude, longitude, ground speed, magnetic heading and wind speed. These different communication words will be transmitted successively on a given bus, but not necessarily in a predictable order. Because of the numerous parameters and associated communication words that may be present on any given bus, it becomes very difficult to analyze the digital data that appears on the bus and, hence, verification of LRU operation, trouble shooting and maintenance commensurately difficult.
To a large extent, these and other related problems are solved by an earlier invention disclosed and claimed in pending U.S. patent application, Ser. No. 227,371, for "APPARATUS FOR TESTING DIGITAL COMMUNICATION BUSES," now U.S. Pat. No. 4,393,498 filed Jan. 22, 1981, by T. Jackson et al. Disclosed in that application is an apparatus that not only receives and analyzes communication words appearing on a subject bus, but also enables test communication words to be generated and transmitted onto a bus for analyzing and verifying the response characteristics of the recipient LRUs. While the test apparatus disclosed in application, Ser. No. 277,371, now U.S. Pat. No. 4,395,498, meets the objectives sought by the inventors of that equipment, nevertheless there is a need for a less sophisticated, cost effective testing apparatus for performing some, but not all, of the test capabilities described in Ser. No. 227,371 now U.S. Pat. No. 4,393,498.
In particular and pertinent to the present invention, there is a need for test equipment that is conceived specially for analyzing communication words that appear or an interconnect bus and, in particular, has the capability of receiving, identifying and extracting (for analysis) individual communication words from the bus on a label-by-label basis. Such analysis may, for example, involve the isolation and decoding of the word data field which contains the engineering, numeric and/or discrete information.
Additionally, it is desirable to provide the capability in such a testing apparatus for receiving, decoding and displaying a list of the labels of all communiction words present on a given bus. Moreover, these capabilities are preferably performed by a test apparatus that makes efficient use of general purpose test equipment, such as a commercially available data analyzer and oscilloscope.